MINERAL, VA., JULY 22 -- A tube section that ruptured at Virginia Power's North Anna nuclear power plant here last week, releasing a small amount of radiation and shutting down the plant at least until next month, was not inspected during a recent refueling of the unit, a utility official said today.

The incident occurred in an area not previously considered a trouble spot either for North Anna or the industry as a whole, William L. Stewart, Virginia Power vice president for nuclear operations, said at a press briefing at the facility.

Company officials had said earlier that all of the tubes in the Unit 1 reactor's three steam generators had been inspected during the April-to-June refueling. Today they said that they had inspected all of the "hot-leg" half of the arch-shaped tubes, where past problems have been identified, but that only 16 percent of the "cold-leg" portion had been tested. The ruptured section was not among those inspected.

"We have a high level of concern because it {the rupture} is in an area that history has shown as not being a hotbed of problems," Stewart said.

As a result of the rupture, North Anna officials are reassessing their practice of inspecting only samples of tubing in the "cold-leg" portion. Cold-leg inspections are not widespread in the industry, because this has not been a problem in the past, a company spokesman said later.

A Nuclear Regulatory Commission official at the plant told reporters it was too early to tell whether the incident at North Anna would lead to a change in inspection practices or standards. In most cases Virginia Power went beyond what it was required to do by the NRC, said Floyd Cantrell, NRC section chief for the Atlanta region, which sent an inspection team to the site after the incident.

NRC officials said that this is only the fifth time that a complete rupture, with a tube section separating completely, has occurred in either the hot or cold legs. Water in the steam generating system goes into the "hot leg" at about 600 degrees and out the "cold leg" at 500 degrees. Each of the plant's three generators has about 3,300 pipes.

All of the cold-leg tubing at the Unit 1 reactor will be inspected during the shutdown, and all of the tubing in Unit 2 will be checked during an imminent refueling, according to utility officials. Unit 1 cannot go back into service until mid-August at the earliest, Stewart said.

NRC records show that Virginia Power officials have had concerns about tube defects in its North Anna Unit 1 steam generators for several years, and that 8 percent of the tubes in the generator with the ruptured tube have been plugged because of known or potential flaws.

But a spokesman said today that those company reports all dealt with problems in the hot-leg side, with the hottest portions showing the most evidence of defects. That evidence steered the company away from concern over the colder sections, he said.

Stewart reiterated that the utility would be able to satisfy customer power demands even with the current heat putting high demands on the system, because the company is buying power from other sources. He backed off an earlier figure of In most cases Virginia Power went beyond what it was required to do by the NRC.

$200,000 to $500,000 a day in added fuel costs as a result of the shutdown, saying the cost could not be assessed until after the unit is operational again.

Officials said they could not estimate how much, if any, of the cost will be passed along to consumers.

Industry critics have said the North Anna incident is another example of aging nuclear power plants experiencing unanticipated and unpredictable problems.